This invention relates to a method for cleaning storage tanks used in the preparation of unsaturated carboxylic acids and their esters.
Unsaturated carboxylic acids and esters of the acrylic acid or methacrylic acid type are currently prepared industrially by heterogeneously catalyzed gas-phase oxidation of the corresponding alkenes, alkanes or unsaturated aldehydes or from the reaction of sulfuric acids and acetone cyanohydrin. Typically, to avoid polymerization during the generation of the desired product, stabilizers such as phenothiazine (PTZ) hydroquinone methyl ether (MeHQ), hydroquinone (HQ), alky and aryl substituted-phenylene diamine derivatives are used. Nevertheless, undesired polymer formation occurs and deposits on the reactors, distillation and rectifying columns, separators and product and intermediate product storage tanks.
The presence of the undesirable solid residue in the storage tanks can lead to contamination in the feed lines along with the downstream equipment causing fouling and inoperability which can greatly affect heat exchangers and reboilers and distillation column efficiencies. Removal of the storage tanks from service for cleaning can be especially expensive and logistically difficult where these tanks are typically used as the bulk material storage during a prolonged shut-down when the plant is being cleaned. In some cases, cleaning and shutdown of the tanks may well require a complete facility shutdown. In the case of unsaturated organic acids and esters the materials are not only flammable and hazardous, they are also difficult to transfer due to the nature of the undesirable polymeric solid residue which typically includes high molecular weight solids. Therefore, it becomes especially important that the storage tank cleanings be done as efficiently and expediently as possible, while maintaining a simple and reliable cleaning method.
U.S. Pat. No. 7,331,354 is directed to a method for cleaning equipment used in the production of methacrylic acid or esters by using a basic liquid. The liquid used for the cleaning process is an aqueous alkali metal and/or alkaline earth metal hydroxide and/or oxide solution, particularly an aqueous solution of NaOH, KOH or Ca(OH)2. The aqueous solution has a dissolved salt content from 0.01 to 30% by weight. However, the use of caustics in a cleaning process is not ideal and may lead to contamination of equipment downstream.
A need exists for an efficient, caustic-free cleaning method for removing solid residue deposited on equipment used in the production of methyl methacrylate (MMA) which takes into account material cost, ease of handling, disposal, and practicality.